For example, in a Cu--Pb bearing alloy which layer comprises Pb-phase grains dispersed in the Cu matrix, the Cu--Pb alloy bearing consisting of a backing metal layer and the bearing alloy layer, the Pb-phase grains are liable to be corroded by acid. Especially in the case where the Cu--Pb alloy bearing exists in an environment of a lubrication oil after a long term use, which oil is called a deteriorated oil, the Pb-phase grains are significantly liable to be corroded by acid substances comprising a chemically changed organic acid (e.g. R-COOH) present in such deteriorated oil. Even if Sn and/or In is further added in the Cu--Pb alloy for improving corrosion resistance property thereof, the Pb-phase grains are not improved in corrosion resistance property since such alloying elements are dissolved merely in the Cu matrix.
Alternatively, there has been known a composite bearing which is provided with an overlay on a bearing alloy layer in order to improve corrosion resistance property as well as conformability, the overlay being Pb-Sn, Pb-Sn-Cu, Pb-Sn-In or the like. This is on the basis of the fact that a Pb alloy comprising 2 to 10 wt % Sn and/or In has good corrosion resistance. It is noted that, if the overlay is directly provided on the bearing alloy layer, Sn and/or In in the overlay migrates into the bearing alloy layer during a heat treatment process or during operation of the engine, for example, as hereby the overlay becomes deteriorated in corrosion resistance property. Thus, the known composite bearing has been provided with an intermediate layer of Ni between the bearing alloy layer and the overlay in order to prevent migration of Sn and/or In in the overlay into the bearing alloy layer. But, since the intermediate layer of Ni is hard, in an internal combustion engine, the pressure of such a Ni layer creates the liability of occurrence of seizure between the composite bearing and the rotary shaft supported by the bearing so as to expose the hard intermediate layer of Ni when the overlay is lost by abrasion after long term operation of the engine. Further, there has been a problem that, when the overlay and a part of the intermediate layer of Ni are lost by early wear to expose the bearing alloy layer due to an uneven contact between the bearing and the rotary shaft at an early stage of engine operation, corrosion starts to progress from the exposed region.
Thus, the present inventors have previously proposed a solution to solve the above problems, which is shown in JP-A-7-179964.
JP'964 teaches a composite bearing in which an overlay of a Pb base alloy comprising 9 wt % Sn and 9 wt % In is formed directly on a Cu--Pb bearing alloy layer without an intermediate Ni layer, and which has been subjected to a diffusion heat treatment at 165.degree. C. for approximately 1,000 hours. According to the diffusion heat treatment, indium (In) in the overlay migrates into Pb-phase grains being dispersed in the matrix of the Cu--Pb bearing alloy layer, thereby the Pb-phase grains are improved in corrosion resistance. Since the composite bearing does not comprise the above hard intermediate Ni layer, even if the overlay is lost by abrasion so as to expose the Cu--Pb bearing alloy layer, rotary shaft being supported by the bearing is not damaged by the bearing without the hard intermediate Ni layer and the Cu--Pb bearing alloy layer has good corrosion resistance because of the In-containing Pb-phase grains.
But, according to the proposal of JP'964, it is required for the diffusion heat treatment to have a long time for indium (In) migration from the overlay into the Pb-phase grains. In order to shorten the heat treatment time, typically the heat treatment temperature is raised or a content gradient is raised between the overlay and the bearing alloy layer. But, in the case where the indium (In) amount of the overlay is raised, it is rather necessary to lower the heat treatment temperature because of a lowered melting point of the overlay comprising an increased amount of indium (In). Thus, according to the prior art of JP'964, it has been required for the diffusion heat treatment to occur over a long time.
Further, according to the proposal of JP'964, it has been difficult to control the indium (In) amount in the overlay, since indium (In) migration from the overlay into the Pb-phase grains during the diffusion heat treatment lowers the indium (In) amount in the overlay.